Zirconium-niobium-nickel cathodes



Z IRCONIUM-NIOBIUM-NICKEL CATHODES A. M. OLSEN BY A TTOR/VE V Nov. 2,1965 H. E. KERN ETAL ZIRCONIUM-NIOBIUM-NICKEL CATHODES 2 Sheets-Sheet 2Filed Aug. 29, 1962 LN $27, n:Q

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ooodm oood .H. E. KERN /Nl/E/vofes. K M OLSEN BV/ 4f M gQTOR/VEV UnitedStates Patent O ZIRCONIUM-NIOBIUM-NICKEL CATHODES Herbert E. Kern,Florliam Park, and Karl M. Olsen,

Madison, NJ., assignors to Bell Telephone Laboratories,

Incorporated, New York, N.Y., a corporation of New York Filed Aug. 29,1962, Ser. No. 220,213 6 Claims. (Cl. 117-223) This invention relates tocathode elements of the indi- -rectly-heated type which are destined foruse in thermionic tubes.

The most conventional type of cathode in commercial use at the presenttime is known as the coated cathode and typically comprises a nickelbase having a coating of alkaline earth metal oxides, generallyincluding barium oxide, and a bind-er which enables the various coatingsto adhere to the cathode base. In addition to the cornponents recited,an activating agent, which performs the function of producing theemission characteristics of the structure, is added to the base member.Among the more effective of such activators is zirconium, which producesemission within about one hour on station, but which begins falling oifafter about 700 hours.

In accordance with the present invention, nickelzirconium-niobiumcathodes evidence an increase in total emission from 1500 to greaterthan 9000 hours on life test and evidence superior tensile strengthproperties as compared with the nickel-zirconium cathodes.

The invention will be more fully understood and other aspects willbecome apparent from the description of the invention, which will bemade with reference to the accompanying drawing, forming a part of thespecification, and wherein:

FIG. 1 is a graphical representation on coordinates of total thermionicemission in amperes per square centimeter against time in hours, showingthe life test data for nickelzirconium, nickel-niobium andnickel-niobium-zirconium cathodes; and

FIG. 2 is a graphical representation on coordinates of tensile strengthin pounds per square inch against temperature of heatiing showing thetensile strength at room temperature of nickel-zirconium, nickel-niobiumand nickel-niobium-zirconium coated cathodes;

FIG. 3 is a cross-sectional view of a nickel-niobiumzirconium cathodeprepared in the described manner.

A general outline of a method suitable for use in the manufacture of athermionic tube in accordance with the method of this invention -is setforth below. Certain operating parameters and ranges as well as the typeof materials employed are indicated.

The cathode base of the present invention is an alloy of nickel, niobiumand zirconium, the latter two being designated activators. The grade ofnickel chosen should be as nearly pure as practical so as not to containany contaminant which may impair the emitting characteristics of thecathode. Any conventonal cathode grade nickel such as carbonyl nickelpowder has been found suitable in this use. It is, likewise, desirableto employ metallic zirconium and niobium which are as pure as practical,such materials being of high pur-ity and obtainable from commercialsources.

Any of the emitting mixtures well known in the preparation of sprayedand molded cathodes may be used in the cathode described herein. Thesematerials contain a barium compound which will be converted to bariumoxide upon thermal decomposition in a vacuum or by some other means as,for example, in a hydrogen atmosphere. Typically, this compound is acarbonate. Among the materials preferred for this purpose are the singlecarbonate material, barium carbonate; the double car- "ice bonatematerial, coprecipitated bariumstrontium carbonate; and the triplecarbonate material, coprecipitated barium-strontium-calcium carbonate.The most commonly available material for this purpose is a coprecipitantof equimolar portions of barium carbonate and strontium carbonate.

In addition to the carbonates listed above, there may be added to theemitting mixture a binder material. The binder is considered to functionas an adherent and suitable materials for this purpose are well known tothose skilled in the cathode art. Common binder materials which willoperate satisfactorily include nitrocellulose or acetone solutions ofstearic acid or isobutylmethacrylate. Binders are added to the mixturein minimum quantities to assure maximum density and to avoid possiblecontamination due to impurities contained therein.

The following is an outline of the procedure to be followed in producinga cathode element from the above materials.

The first step in the preparative technique involves preparing thecathode base which is an alloy of nickel, zirconium and niobium. To thisend, carbonyl nickel powder obtained from commercial sources andinitially containing minimum quantities of oxygen and carbon issubjected to wet hydrogen reduction, thereby lowering the concentrationof oxygen and carbon to a satisfactory level.

The reduction is effected by sintering a desired quantity of powder,typically of the order of 25 pounds, in a suitable container, such asmagnesium oxide, at temperatures of the order of 800 C. in the presenceof wet hydrogen for a time period within the range of l2 to 16 hours.

Alloying is conducted in a suitable furnace, such as a controlledatmosphere induction furnace. These furnaces typically include a centralcrucible contained within an induction coil and a bin and chute foradditions to the melt. The Crucible employed is preferably one fromwhich minimum contamination might be expected, magnesium oxide havingbeen found suitable in such use.

The hydrogen reduced nickel slugs are next charged into the magnesiumoxide Crucible and the alloying agent comprising essentially purezirconium and niobium are placed in the addition hopper of the furnace,after which the furnace is closed. The zirconium is added in an amountwithin the range of 0.04 to 0.05 percent by weight based on the weightof the total alloy composition. The addition of zirconium in amountsappreciable below the indicated minimum result in a slower rate ofactivation than is considered practical and a shorter life for theresultant structure. The upper limit of 0.5 percent is dictated byconsiderations of the solubility of zirconium in nickel. Niobium isadded in an amount within the range of 0.1 to 2 percent by weight basedon the total w-eight of the alloy. Although the indicated minimumpercentage of niobium is not absolute, it will be understood that suchamounts are considered necessary to produce a noticeable eifect upon theproperties of the resultant structure, whereas exceeding the indicatedmaximum produces no further beneficial effect.

Next, the system is evacuated to a pressure of approximately lmillimeter of mercury by means of a mechanical pump. Dry hydrogen havinga dew point of the order of -20 F. is then introduced into the systemuntil a continuous ow of approximately 20 cubic feet per hour at oneatmosphere of pressure is attained. Then, the nickel charge is heatedinductively using a suitable generator, such as a 1920 cycle, 450 volt,killowatt generator, to a temperature within the range of 1455 to l650C. Complete melting is found to occur in approximately one hour. Inorder to assure complete miscibility,

the molten charge is held an additional minutes in dry hydrogen.

After melting, the dry hydrogen is purged from the system with dryhelium, the system re-evacuated to a pressure of the order of onemillimeter anddry hydrogen again introduced in order to further reducethe oxygen and carbon content. Following, the system is again ushed withdry helium and evacuated.

Helium is now reintroduced into the system and the zirconium and niobiumadded to the molten nickel from the addition hopper by means of a chuteleading directly into the magnesium oxide crucible.

Finally, the molten charge is poured under dry helium maintained at apressure of approximately one atmosphere and a temperature of 1500 to1600 C. into an alundum-coated steel mold and permitted to cool.

The resultant ingots are then hot rolled, annealed and mach-ined byconventional metallurgical techniques.

The base material having been completed, the next step in thepreparation of a cathode involves machine spraying the formed cathodewith any of the noted emitting mixtures, for example, a triple carbonatecontaining 1% by weight of nitrocellulose, 49% BaCo3, 44% SrCo3 and 7%CaCo3. The coating area may typically be 0.05 cm.2 with a coatingdensity of 2.0 gm./cm.3 and a thickness of 0.5 mil or a coating densityof 1.0 gm./cm.3 and a thickness of 1.5 mils. The resultant cathodestructure is shown in a cross-sectional view in FIG. 3. All that remainsin the manufacture of a usable cathode -is to assemble the element in atube envelope, convert the carbonates to the oxides and seal the tube.Since this procedure is well known to those skilled in the art, it willnot be described in detail. In brief, the procedure involves sealing atube structure containing the element on a vacuum station which isevacuated to a pressure of the order of 10-Fl millimeters of mercury.The cathode is then heated at a temperature of the order of 950 C. untilthe carbonates are broken down to the oxides. This heating procedure,which may take of the order of zero to 30 minutes for an emitting layerthickness of approximately 1-4 m-ils is terminated when substantiallyall of the carbonates are converted to oxides. The breakdown point isindicated by a sudden drop in pressure within the chamber. The cathodeis then heated to labout 1000 C. and is held at this temperature forabout 2 minutes. Finally, the temperature of the structure is dropped toabout 740 C. and total current measured by pulse measurement.

In order that those skilled in the art may more fully understand the-inventive concept herein presented, the following example is given byway of illustration and not limitation.

Example I 25 pounds of carbonyl nickel powder obtained from commercialsources and having an oxygen content of 0.2 percent by weight and acarbon content of 0.07 percent by weight were inserted into a magnesiumoxide crucible which was inserted in a nichrome pot through which wethydrogen flowed continually at a temperature of 800 C. in a mufetypefurnace for 14 hours, so reducing the carbon and oxygen content to lessthan 0.01 percent by weight.

The resultant slugs were next charged into a magnesium oxide Cruciblecontained in a controlled atmosphere induction furnace containing anaddition hopper to which there was added 0.25 pound of metallic niobiumobtained from commercial sources and having a purity of 99.94- percentand 0.025 pound of metallic zirconium obtained from commercial sourcesand having a purity of 99.84- percent. The furnace was then closed andthe system evacuated by means of a mechanical pump to a pressure of onemillimeter of mercury;

Dry hydrogen having a dew point of 20 F. was then admitted to the systemuntil a continuous flow of 20 cubic feet per hour was established. Thefurnace was next heated to a temperature of 1550 C. and maintained atthat level for 70 minutes to melt the contents.

Following heating, the system was pumped with dry helium (dew pointapproximately -20 F.), re-evacuated to a pressure of one millimeter ofmercury, and dry hydrogen readmitted to the system.

Next, the system was flushed with dry helium, evacuated again and dryhelium readmitted. At this stage, the zirconium-niobium mixturecontained in the addition hopper was added to the moltenmixture. Theresultant charge was then poured under dry helium maintained at apressure of one atmosphere at 1550 C. into an alundum-coated steel moldand permitted to cool.

The resultant ingot, having a composition of 1% by weight Nb, 0.1% byweight Zr, remainder Ni, was then removed from the mold and hot rolledin air at 1000 C. to 1/2 inch plate, the surfaces machined to removeoxide scale and cold rolled to 0.020 inch strips with an intermediateanneal at 0.080 inch in hydrogen at 800 C. Several sample lengths werere-annealed and rolled further to 0.003 inch, annealing being conductedby continuously passing the strip through a heated muflie at 1.5 feetper minute.

The completed base material was then machine sprayed with a triplecarbonate containing 49% by weight barium carbonate, 44% by weightstrontium carbonate and 7% by weight calcium carbonate, and 1% by weightnitrocellulose binder. The coating'area was 0.05 cm.2 and had a densityof 2.0 gn1./cm.3 and a thickness of 0.5 mil.

The cathode element was then sealed on a vacuum station and evacuated toa pressure of 107 millimeters of mercury, heated to 950 C. forapproximately 10 minutes, after which the temperature was increased to1000 C. for 2 minutes. The temperature of the structure was then droppedto 740 C. and total current measured by pulse measurement.

For comparative purposes, two cathode elements were prepared in themanner described above, one containing 0.1% zirconium, remainder nickeland one containing 1% niobium, remainder nickel.

Referring again to the figures, FIG. l is a graphical representation oftotal emission in lamperes per square centimeter against time inthousands of hours showing total emission data for nickel cathodescontaining 0.1% zirconium, 1% niobium and 0.1% zirconium and 1% niobiumprepared in accordance with the procedure described in Example I. Totalemission is ploted against time and is measured by applying a 400 volt,61u4 second square wave with a repetition rate of one pulse per second.The pulse is superimposed directly over the steady state direct-currentoperating conditions. The cathode of the present invention activates to4.35 amperes per square centimeter in about 400 hours and continues athigh levels of emission, after falling ott slightly at 1500 hours, forover 9000 hours. The nickel niobium cathode activates to 3.2 amperes persquare centimeter in about 400 hours, continues at that level until 1500hours and gradually drops off to terminate life after about 7000 hours.The nickel-zirconium cathode activates to about 7 amperes per squarecentimeter in about 700 hours and continues at high levels of emissionfor over 10,000 hours although gradually dropping oil. Thus, it is seenfrom the curves that the cathode of the present invention activates morerapidly than the conventional nickel-zirconium cathode and comparesfavorably with the emission characteristics of that structure,evidencing an upward trend after 9000 hours as compared with thedownward trend of the nickel-zirconium. Further, it is noted that theemission levels of the subject cathode are far superior to thenickelniobium structure.

FIG. 2 is a graphical representation of tensile strength in pounds persquare inch measured at room temperature plotted as a function ofcathode heating temperatures for the three structures described withreference to 5 FIG. 1. The zirconium-nickel cathode evidenced tensilestrengths ranging from 120,000 pounds per square inch before heattreatment to 113,000 pounds per square inch After heating temperaturesup to 400 C. and sharply dropped on to a level of 57,000 pounds persquare inch after heating to 1000 C. The niobium-nickel cathodeevidenced tensile strengths varying from 135,000 pounds per square inchbefore heat treatment down to 115,000 pounds per square inch afterheating to 600 C. before dropping off sharply to 63,000 pounds persquare inch after heating to 10000 C. The cathode of the presentinvention rather than evidencing properties intermediate the otherstructures evidenced tensile strengths ranging from 137,000 pounds persquare inch before heat treatment down to 116,000 pounds per square inchafter heating to 600 C. before dropping off to 63,000 pounds per squareinch after heating to l000 C. Thus, it is noted that the subject cathodeunexpectedly shows superior metallurgical properties. The data plottedwas obtained by measuring tensile properties of a wire of the subjectmaterial after a 1/2 hour heat treatment in hydrogen at the indicatedtemperatures followed by cooling to room temperature.

While the invention has been described in detail in the foregoingspeciication and drawing, it will be appreciated by those skilled in theart that variations may be made without departing from the spirit andscope thereof.

What is claimed is:

1. A cathode element including an alloy consisting 6 essentially of0.12% by weight niobium, 0.04-0.5% by weight zirconium, remaindernickel.

2. A cathode element including an alloy consisting essentially of 0.1%by weight zirconium, 1% by weight niobium, remainder nickel.

3. A cathode element destined for use in a thermionic tube comprising abase member, an emissive coating and a binder, said base memberconsisting essentially of an alloy of 0.1-2% by Weight niobium,0.04-0.5% by weight zirconium, remainder nickel.

4. A cathode in accordance with claim 3 wherein said emissive coating isselected from the group consisting of barium carbonate, bariumstrontiumcarbonate and barium-strontium-calcium carbonate.

5. A cathode in accordance with claim 3 wherein said base member is analloy consisting essentially of 0.1% by weight zirconium, 1% by Weightniobium, remainder nickel.

6. A cathode in accordance with claim 5 wherein said emissive ycoatingis barium-strontium-calcium carbonate.

References Cited by the Examiner UNITED STATES PATENTS 2,833,647 5/58Hoff et al. 75-170 2,938,785 5/60 Bounds et al. 75-170 2,985,548 5/61Blickwedel et al. 117-221 3,088,851 5/63 Lemmens et al 117-221 RICHARDD. NEVIUS, Primary Examiner.

3. A CATHODE ELEMENT DESTINED FOR USE IN A THERMIONIC TUBE COMPRISING ABASE MEMBER, AN EMISSIVE COATING AND A BINDER, SAID BASE MEMBERCONSISTING ESSENTIALLY OF AN ALLOY OF 0.1-2% BY WEIGHT NIOBIUM,0.04-0.5% BY WEIGHT ZIRCONIUM, REMAINDER NICKEL.